The present invention relates to the discovery that alkyltin hydrides display selectivity with respect to both (a) the preferential reduction, under mild conditions, of one halogen to the exclusion of another halogen in a mixed halosilane and to (b), the stepwise reduction of halosilanes.
Substitution of hydrogen for chlorine in polychlorosilanes such as (CH.sub.3)SiCl.sub.3 is known and can be effected via reaction with trialkylsilanes in the presence of catalytic quantities of aluminum chloride. However, this catalyst is notoriously effective in promoting skeletal rearrangements (i.e., redistributions involving Si-C and/or Si-Si bonds).
Organotin hydrides have long been known as effective reducing agents in organic chemistry and their reactivity patterns have been well documented. Perhaps the most common use of tin hydrides is in the reduction of organic halides. EQU (4-n)RX+R'.sub.n SnH.sub.4-n .fwdarw.(4-n)RH+R'.sub.n SnX.sub.4-n
The above reaction has a wide scope: R can be alkyl, aryl, acyl, propargyl, etc. The reaction conditions will vary with the R group (eg., aryl halides are more difficult to reduce than alkyl halides) and with the halide (for alkyl halides the order of reactivity is RI&gt;RBr&gt;RCl&gt;&gt;RF). These reactions proceed through free radical mechanisms and require the addition of free radical initiators.
U.S. Pat. No. 4,617,357, issued Oct. 14, 1986 to Pallie et al., teaches the reduction of residual chlorine in glycidyl compounds with certain organo-tin hydrides without the glycidyl group thereby being attacked.
Aono et al., U.S. Pat. No. 4,639,361, issued Jan. 27, 1987, teaches a process of preparing disilane Si.sub.2 H.sub.6 from the reduction of hexachlorodisilane Si.sub.2 Cl.sub.6 with a mixture of LiH and LiAlH.sub.4.
Hiiro et al., U.S. Pat. No. 4,115,426, issued Sept. 19, 1978, teaches a method of preparing dialkylchlorosilanes by the selective dechlorination reduction of a dialkyldichlorosilane. The reducing agents in the preparation are sodium borohydride and sodium hydride.
Grady, et al., J. Org. Chem., vol. 34, p. 2014-2016, 1969, teaches the use of organotin hydrides for the stepwise free radical reduction of organic halides, such as geminal polyhalides, and simple carbonyl compounds. Grady et al. does not teach the selective stepwise or other reduction of halosilanes, nor does it teach reduction methods in the absence of free radical mechanisms. Similarly, Wilt, et al., J. Am. Chem. Soc., vol. 105, p. 5665-5675, 1983, teaches the reduction of a silicon-containing organic halides by organotin hydrides, but is limited to the reduction of alpha-halosilanes, i.e., the halogen atom is bonded to a carbon atom adjacent to a silicon atom but is not bonded directly to a silicon atom.
Organotin hydrides typically react with organohalides and unsaturated bonds via a free radical mechanism. These reactions either require heating or initiation by a free radical source.
U.S. Pat. No. 3,439,008, issued Apr. 15, 1969 to Berger, teaches and claims a method of reacting an organostannane and a halogenated Group IVb elemental material selected from the group consisting of a halosilane, a halogermane, and a halosiloxane. However, Berger requires the presence of a Group III metal compound promoter and does not teach or claim the selective, stepwise reduction of halosilanes.
There exists a need for a preferential reduction of one halogen species to the exclusion of another in molecules such as SiF.sub.2 Br.sub.2 and other halomonosilanes.